The present invention relates to methods and systems for protecting motor/compressors in refrigeration systems, including air conditioners and heat pumps, against excessive loading caused either by a high operating load or by insufficient condenser airflow, as these terms are hereinbelow defined. In preferred embodiments the protection methods and systems of the present invention may be termed "generic" in that a single system is capable of serving a number of different models, of widely differing capacities.
The invention is generally applicable to refrigeration systems of the type employed in air conditioners and heat pumps for cooling and heating living spaces. Such units are available in a wide variety of physical configurations and capacities, two of which are small room air conditioners and self-contained reversible heat pump systems. The latter somewhat resemble room air conditioners, but provide both heating and cooling. For convenience, the invention is described herein in exemplary forms applied to a simple room air conditioner. The principles of the invention are also applicable to similarly-configured heat pump systems which provide both heating and cooling by means of a reversible refrigeration system, as well as central air conditioning systems which employ an indoor evaporator and a separate outdoor compressor/condenser combination.
Such refrigeration systems, while apparently simple to control, in fact require fairly sophisticated control systems if proper operation and protection from damage over a wide variety of operating conditions, often adverse, are to be achieved.
A basic form of protection for a refrigerant motor/compressor is overload protection, and is typically provided by a thermal or overcurrent sensor. By way of more specific example, various motor and compressor protection systems are disclosed in the following U.S. patents: Anderson et al. U.S. Pat. No. 4,038,061; Godfrey U.S. Pat. No. 4,079,432; Newell U.S. Pat. No. 4,253,130; and Genheimer et al. U.S. Pat. No. 4,286,303. Of these, Anderson and Newell disclose relatively comprehensive systems for protecting air conditioners and heat pumps, and employ a variety of current and temperature sensors. Godfrey and Genheimer et al disclose motor protection systems in general which include the function of allowing a motor to attempt a restart following an overload, but only for a limited number of times.
Another approach to motor protection, particularly for a refrigeration system compressor motor, is disclosed in commonly-assigned Pohl U.S. Pat. No. 4,196,462. As disclosed in that patent, a single-phase AC induction motor of the type employing a capacitor-run winding can be protected from overload (including locked-rotor) conditions by monitoring the voltage across the capacitor-run winding. Under heavy loading conditions, the winding voltage decreases. This can be sensed, and used to initiate appropriate protection measures, such as a timed cooling-off interval.
While not prior art with respect to the present invention, it may be noted that related, but more sophisticated, protection systems and methods are disclosed in two commonly-assigned U.S. patent applications. Specifically, these are Ser. No. 778,076 filed, Sept. 20, 1985, by Walter J. Pohl, entitled "Self-Calibrating Control Methods and Systems for Refrigeration Systems" now U.S. Pat. No. 4,653,285; and Ser. No. 778,075, filed Sept. 20, 1985, by Walter J. Pohl, entitled "Protection Methods and Systems for Refrigeration Systems Suitable for a Variety of Different Models"; the entire disclosures of which are hereby expressly incorporated by reference.
Very briefly, the systems described in application Ser. No. 778,076 sense loading on the compressor motor, preferably by sensing the voltage across the capacitor-run winding of an AC induction motor and normalizing with respect to line voltage. A self-calibrating protection capability is implemented by utilizing the changing load as a function of time characteristic on the compressor motor during normal and abnormal operation of a refrigeration system. More particularly, a reference value of compressor motor loading is determined and stored shortly after the start of each compressor ON cycle by allowing a stabilization interval (typically thirty seconds) to elapse, and then sensing loading and storing the sensed loading as the reference value to be used for the remainder of that particular ON cycle. In the preferred forms, it is the ratio of capacitor-run winding voltage to line voltage which is sensed and stored as a reference ratio. Thereafter, during each particular ON cycle, in order to recognize high load conditions, prevailing compressor loading is at least periodically sensed and compared to the stored reference. If the thus-sensed motor loading has increased above a high-load threshold, then a high load condition is recognized, and the compressor motor is de-energized for a timed cooling off interval. In the preferred forms, it is then-prevailing ratio of capacitor-run winding voltage to line voltage which is sensed and compared to the stored reference ratio. The compressor motor is de-energized if the then-prevailing ratio falls below a high-load threshold ratio established as a predetermined fraction of the reference ratio, typically 0.8 times the reference ratio. The approach disclosed in Ser. No. 778,076 can be made self-calibrating, and compressor motor protection afforded regardless of the size of the motor, since the motor control system establishes its own reference based on the characteristics of the particular motor. In this regard, the systems may be characterized as "generic".
The systems described in application Ser. No. 778,075 similarly sense loading on the compressor motor, but the reference for comparison purposes is not self-determined at the start of each compressor ON cycle. Rather, a permanent reference is established for each particular system in the factory, when the system is new, operating with a known correct refrigerant charge, and under a known load. The permanent reference is established after a timed stabilization interval has been allowed to elapse during which start-up transients, liquid slugging effects, and the like have dissipated, but before the compressor is significantly loaded as a result of pressure build-up. An advantage of the approach of Ser. No. 778,075 is that it permits a loss of refrigerant condition to be detected. While a calibration step is required, the technique nevertheless may be characterized as "generic" in the sense that a single control system may be employed in a variety of different air conditioner or heat pump models, without being specifically tailored for a particular model. Moreover, protection is afforded without the need for providing a variety of sensors.
Among the adverse conditions which can be detected by the various systems and techniques referred to above is a condition of excessive loading on the refrigerant motor/compressor, also referred to herein simply as a "high load" condition. There are two general categories of "high load" conditions, and these are herein referred to as a "high operating load" and "insufficient condenser airflow", respectively.
A "high operating load" can result when power line voltage is excessively low (a so-called "brown out" condition), or when operating under extreme ambient temperature conditions. On an extremely hot day, an air conditioning system may be subjected to both high load and low voltage. This tends to make the motor inefficient, which leads to overheating. Under such operating conditions, it is desirable to de-energize the compressor before damage results, then allow operation to resume after a cooling-off interval. Thus, the various systems referred to above typically respond to a high load condition by temporarily de-energizing the compressor motor for a cooling-off interval determined by either time or temperature. Operation then resumes until another cooling-off interval is initiated. In this way, the refrigeration system, specifically an air conditioner, is operated at its maximum capacity consistent with existing operating conditions.
It may be noted that a locked rotor condition is different from a "high operating load" as the term is employed herein, (although not all systems make a distinction). In the systems of the above-incorporated application Ser. Nos. 778,075 and 778,076, for example, a locked rotor condition is recognized by an exceptionally low capacitor-run winding voltage after an equilibrium speed interval has elapsed, a few seconds after the compressor has been energized. Failure of the compressor motor to start at all is usually due to an attempt to restart the compressor before pressures within the closed-circuit refrigeration system have had time to equalize. Normally, after a further delay, the compressor can be successfully started. However, if after several attempts the compressor does not start, a more serious problem is indicated. Accordingly, the systems of the above-incorporated application Ser. Nos. 778,075 and 778,076 each maintain a "locked-rotor count" and terminate operation entirely if the "locked-rotor count" exceeds a predetermined number, for example six. In the event a successful start occurs, recognized by, for example, three minutes of continuous run time, the "locked rotor count" is reset.
The other general category of "high load" conditions referred to above is an "insufficient condenser airflow" condition, which is sometimes referred to as a "blocked condenser". This condition can be caused by the condenser fan not rotating, or by sand or dust clogging up the condenser heat-exchange surfaces. Under such conditions, excessive refrigerant pressures build up in the system, and the compressor overheats. This is a condition which will not remedy itself, and treating it the same as a "high operating load" will likely lead to eventual damage.